The present invention relates to solar concentrators, and more particularly to concentrators capable of harnessing light without accurate tracking due to the use of total internal reflection.
Existing solar concentrators use lenses and reflectors that are very sensitive to angle of incidence, requiring accurate tracking with all its drawbacks. These systems can""t harness diffuse light or feed light from cheap reflectors to refractive interfaces, and typically cannot achieve concentration ratios to compete with conventional utilities.
The present invention provides a solar energy system that achieves concentration for incident solar energy without requiring accurate solar tracking or high precision reflectors. This concentration is achieved by total internal reflection (xe2x80x9cTIRxe2x80x9d) within a tapering refractive element, which, according to Snell""s law, will produce, from the refractive element, a smaller exit angle than the corresponding angle of incidence.
The tapered TIR element may be, for example, a wedge, a cone, polygonal pyramid, irregular or asymmetrically shaped refractive object. It is preferred that the internally reflective faces of the TIR element be smooth, and that the TIR element taper to a sharp tip, although embodiments of the invention do not all require such conditions. In addition, the refractive index of the TIR element may be uniform or graded, or the TIR element formed as a composite of solids or solids and liquids, with the same or different refractive indices. The surfaces of the TIR element may also be coated.
The TIR refractive element typically receives light through an interface, which is typically a refractive interface with air. Preferably, the difference in refractive index between the TIR refractive element and air is great, for example the refractive element has a refractive index of at least 1.5, and preferably 1.7 or greater. The index of refraction of air is about 1.0. The light, after entering the TIR refractive element, passes through the refractive medium, and, according to Snell""s law, will internally reflect at each interface wherein the angle of incidence is below a critical angle and will pass through the interface wherein the angle of incidence is greater than the critical angle. Because the TIR refractive element tapers, the edge interfaces are inclined with respect to each other. Therefore, the critical angles for the respective edge interfaces are less than 180xc2x0 apart. This results in a relative convergence of the rays (as compared to the incident radiation) toward a projection of a line of symmetry between the respective edge interfaces.
Light entering a transparent wedge or conical element thus remains in total internal reflection until a critical angle is reached, at which point the light exits, generally pointing in the direction of a xe2x80x9cfocusxe2x80x9d and having an exit xe2x80x9cspreadxe2x80x9d or xe2x80x9cconexe2x80x9d which is dependent on the shape and index of refraction of the TIR element. Higher indices of refraction and gradual (narrow angle) tapers generally yield narrower exit spreads, according to Snell""s law.
By providing a gradual taper, internal reflections off the converging faces will gradually approach the critical angle, so that when the internally reflected light exits, it will be at or near the critical angle, and when light reflects internally it will proceed toward the tip and not reflect back toward the entrance aperture.
In theory, a material with an index of refraction of 1.7 would have a critical angle of about 36xc2x0 and an exit angle of about 88xc2x0, pointing the light generally toward the focus, over a wide range of incident light angles. In practice, an exit xe2x80x9cspreadxe2x80x9d is observed. Therefore, the TIR element achieves xe2x80x9crough collimationxe2x80x9d of the incident light. The xe2x80x9croughly collimatedxe2x80x9d light produced by the TIR element can be focused to high light concentrations by many mechanisms, and therefore a substantial benefit is achieved as compared to known tracking solar collector systems.
Various embodiments of the invention provide a plurality of TIR elements in parallel. Since each TIR element roughly collimates light, these plurality of TIR elements may have converging foci. The incident face, or xe2x80x9cmain refractive interfacexe2x80x9d of these plurality of TIR elements may be planar or, for example, have an overall convex curvature. The roughly collimated light from one or more TIR elements may be further concentrated with additional TIR elements in series, by lenses and/or reflectors.
Four functions describe the ideal main refractive interface: First, rays already heading toward the focus should pass with minimal deviation. Second, some portion of the collected rays are refracted and exit after a small number of xe2x80x9cbouncesxe2x80x9d at or near the critical angle. Third, those rays not naturally falling into categories 1 or 2 above should be gently corrected so as to have a minimal number of internal reflections, but still exit as closely as possible to the desired critical angle. Fourth, those rays exiting the main refractive interface but not heading toward the focus should be in a narrow enough spread to be redirected by cheap secondary optics (e.g. reflective troughs).
The concentrated solar energy may be used in known manner, for example for illumination, heating, photovoltaic conversion, generation of combustible gasses, thermo-mechanical conversion (steam engines, Stirling cycle engines, etc.), photoprocessing of materials, etc. The present invention also provides new embodiments of solar energy conversion systems.
It is therefore an object of the present invention to provide a solar collector comprising an elongated light guide, having an axis, the light guide comprising a high optical refractive index liquid or gel material and a boundary separating the high optical refractive index liquid or gel material from a relatively lower optical refractive index material, and an entrance aperture structure adapted to receive incident light substantially deviating from the axis, wherein the high optical refractive index liquid or gel material of the elongated light guide concentrates light received through the entrance aperture and provides light concentration as compared to incident optical density on the entrance aperture; and a photoelectronic solar transducer for employing energy from the incident light, immersed within the liquid.
It is also an object of the present invention to provide a solar collector comprising a tapered elongated light guide and an axis, comprising a high optical refractive index material and a boundary separating the high optical refractive index material from a relatively lower optical refractive index material, and an entrance aperture adapted to receive incident light substantially deviating from the axis, wherein the high optical refractive index material of the elongated light guide transmits and concentrates light received through the entrance aperture by a process of total internal reflection, the tapered elongated light guide having a taper angle dependent on a ratio of the high optical refractive index to the relatively lower optical refractive index.
It is a further object of the present invention to provide a solar collector comprising an elongated light guide, having an axis, the light guide comprising a high optical refractive index material and a boundary separating the high optical refractive index material from a relatively lower optical refractive index material, and an entrance aperture structure adapted to receive incident light substantially deviating from the axis, wherein the high optical refractive index material of the elongated light guide concentrates light received through the entrance aperture and provides light concentration as compared to incident optical density on the entrance aperture, at least a portion of the light exiting into the relatively lower optical refractive index material; and a reflector for recovering light from the relatively lower optical refractive index material.
It is also an object of the invention to provide a solar collector comprising a main refractive interface, adapted for wide angle reception of solar radiation, and a set of projections from the main refractive interface, each narrowing an exit angle of the received solar radiation from the angle of reception by a process of total internal reflection.
It is another object of the invention that the elongated light guide may be configured such that incident light substantially deviating from the axis received through the entrance aperture undergoes successive internal reflections in the high optical refractive index material, each reflection having a successive angle of incidence on an interface between the high optical refractive index material and the relatively lower optical refractive index material until a critical angle is reached, whereupon the light is transmitted through the interface into the relatively lower optical refractive index material at a smaller angle with respect to the axis than the incident light.
Suitable solar receptors for employing energy from the incident light include a photovoltaic transducer, photohydrolytic transducer, a heat engine, a light pipe and a photothermal receptor.
In a preferred embodiment, the main refractive interface is stationary, but may also crudely track solar movement or be associated with elements, either before or after the total internal reflective element, which crudely track solar movement.
The total internal reflection element is preferably a high index of refraction element having smooth tapering internal reflection faces that meet at a sharp junction.
It is a further object of the invention to provide a solar collection method for using the solar collector comprising the steps of receiving solar radiation through the main refractive interface, and internally reflecting at least once, at least a portion of the received solar radiation, in a set of projections from the main refractive interface to narrow an exit angle of the received solar radiation therein.
It is a still further object of the invention to provide a method of fabricating a TIR solar collector, comprising molding a main refractive interface and set of projections as a planar sheet and vacuum forming the planar sheet into a non-planar sheet.